Acharya Prashant explains the fundamental difference between reaction and response, defining reaction as a biological reflex rooted in animalistic instincts for physical security. These instincts, which are millions of years old and embedded in our DNA, bypass thought and consciousness to ensure survival. While these reflexes were necessary for survival in the jungle, they are often inappropriate for modern human life. He emphasizes that reactiveness is a mechanical, chemical process that lacks consciousness and understanding, effectively turning a person into a predictable and controllable machine.